In particular, the invention can be applied for display as regards the embodiment of active matrix type liquid crystals flat screens and can be used in data processing applications for the embodiment of dynamic RAM type memories (direct access memory).
So as to set forth the problems able to be resolved by the invention, first of all there follows an example of active matrix liquid crystals display screens. However, it goes without saying that this example is not restrictive and RAM type memories are not excluded from the invention.
There currently exist a large number of types of liquid crystal display screens. The screen to be embodied envisaged by the present invention is an active matrix type screen. One advantageous embodiment method is the two masking level method referred to in the document FR-A-2 533 072. This method makes it possible to associate with each pixel a thin film transistor (TFT) whose grid is connected to an addressing line, the source being connected to a column and the drain to the electrode of the pixel.
One active matrix display screen is also described in the article entitled "SOI TFT's with Directly Contacted ITO" published by AKIO MIMURA et al. in IEEE Electron Device Letters, vol. EDL-7, n.degree. 2, February 86, pages 134-136.
There also currently exist other screens in which the switching element is not a TFT but a non-linear element. The general diagram of such a device is shown on FIG. 1. This figure shows the addressing columns Cj and the capacitors Cij having a first armature or electrode el, this electrode being connected to the adjacent column through a non-linear element Sij, all the above being disposed on a first plate (not shown). The second armatures or counter-electrodes cel are connected to addressing lines Li borne by a second plate (not shown).
The application of selected voltages to the lines Li and the columns Cj makes it possible to charge or discharge the capacitor Cij through the element Sij. The resultant electric field makes it possible to control the optical state of the liquid crystal present between the two plates and thus display one point ("pixel") of an image.
The document GB-A-2 091 468 describes such a device and an embodiment method where the non-linear element Sij is an MIM component, that is a metal/insulant/metal stacking.
In the document GB-A-2 091 468, the MIM components are obtained by depositing a first metallic film, anodization producing a film of oxide, followed by the depositing of a second metallic film on the oxide.
Such a technique has a large number of drawbacks:
first of all, this technique requires four masking levels and photolithoengraving, which heavily adversely affects production efficiency which exponentially decreases with the number of levels,
then, recourse to the anodic oxidation phenomenon requires handlings of the support plate and poses serious problems when this involves obtaining thicknesses of homogeneous insulants of about from 10 to 100 nm, this being the case when the Schottky or Poole-Frenkel thermoionic effect recommended in the document is used; in fact, the homogeneity of thickness, that of the dielectric and physical qualities and that of the porosity of the anodic oxide film depend on the uniformity of the current density during anodic oxidation; this density itself is a function of the relief of the sub-jacent films (point effects), the electrolyte used, its impureties, its temperature, the pH, agitation and ageing of the bath; in particular, any electrochemical impurety of the electrolyte, especially the anions contained in the bath, are likely to be trapped in the oxide, which results in risks of eletrical disruptive breakdown and conductive instabilities,
finally, the conduction mechanism linked to the Schottky or Poole-Frenkel effects in the thick (10 to 100 nm) oxide films exponentially depends on the temperature, which renders it necessary to provide an accurate thermal adjustment of the final device, this adjustment proving to be delicate and expensive.